Composite (or ditabase) is a compound DITA document type. The <dita> element is a top-level container for multiple topics of different types ( generic topic, task, concept, reference and glossary), which can go in any sequence.
Figure 1. DITA Composite opened in Serna.
You can find more information about DITA composite topics at: http://docs.oasis-open.org/dita/v1.1/CD01/langspec/langref/dita.html.